Active matrix liquid crystal display (AMLCD) devices are typically composed of a matrix of liquid crystal pixels arranged horizontally in rows and vertically in columns. Such devices typically include first and second opposing polarizers, a liquid crystal layer disposed between the polarizers, and substantially transparent electrodes mounted on opposite sides of the liquid crystal (LC) layer so as to selectively energize same in order to create an image for a viewer.
Electronic matrix arrays find considerable applications in AMLCDs. Such AMLCDs generally include X and Y (or row and column) address lines which are horizontally and vertically spaced apart and cross at an angle to one another thereby forming a plurality of crossover points. Associated with each crossover point is an element (e.g. pixel) to be selectively addressed. These elements in many instances are liquid crystal display pixels or alternatively the memory cells of an electronically adjustable memory array.
Typically, a switching device such as a thin film transistor (TFT) is associated with each array element or pixel. The isolation device permit the individual pixels to be selectively addressed by the application of suitable potentials between respective pairs of the X and Y address lines. Thus, the TFTs act as switching elements for energizing corresponding pixel electrodes.
Amorphous silicon (a-Si) TFTs have found wide usage for isolation devices in LCD arrays. Structurally, TFTs generally include substantially co-planar source and drain electrodes, a semiconductor material (e.g. a-Si) disposed between the source and drain electrodes, and a gate electrode in proximity to the semiconductor but electrically insulated therefrom by a gate insulator. Current flow through the TFT between the source and drain is controlled by the application of voltage to the gate electrode. The voltage to the gate electrode produces an electric field which accumulates a charged region near the semiconductor-gate insulator interface. This charged region forms a current conducting channel in the semiconductor through which current is conducted.
Typically, pixel aperture ratios (i.e. pixel openings) in non-overlapping AMLCDs are only about 50%. As a result, either display luminance is limited or backlight power consumption is excessive, thereby precluding or limiting use in portable applications. Thus, it is known in the art that it is desirable to increase the pixel aperture ratio or pixel opening size of LCDs to as high a value as possible so as to circumvent these problems. The higher the pixel aperture ratio (or pixel opening size) of a display, the higher the display transmission. Thus, by increasing the pixel aperture ratio of a display, transmission may be increased using the same backlight power, or alternatively, the backlight power consumption may be reduced while maintaining the same display transmission.
Currently, a common way of making AMLCDs is to pattern the ITO pixel electrodes at a distance or gap of about 5-10 .mu.m from the bus lines which results in the LC material in this gap area not being activateable. Thus, the black matrix is required on the passive plate to overlap the pixel electrodes by about 5-10 .mu.m so as to avoid light leakage in these areas and to compensate for potential plate misalignment. Thus, there exists a need in the art to eliminate this problem while simultaneously increasing pixel size.
For example, "High-Aperture TFT Array Structures" by K. Suzuki discusses an LCD having an ITO shield plane configuration having a pixel aperture ratio of 40% and pixel electrodes which overlap signal bus lines. An ITO pattern in Suzuki located between the pixel electrodes and the signal lines functions as a ground plane so as to reduce coupling capacitance between the signal lines and the pixel electrode. Unfortunately, it is not always desirable to have a shield electrode disposed along the length of the signal lines as in Suzuki due to production and cost considerations. The disposition of the shield layer as described by Suzuki requires extra processing steps and thus presents yield problems. Accordingly, there exists a need in the art for a LCD with an increased pixel aperture ratio which does not require an ITO shield plane structure to be disposed between the signal lines and pixel electrode.
FIG. 1 is a side elevational cross-sectional view of prior art linear thin film transistor (TFT) 100 of U.S. Pat. No. 5,055,899. A plurality of TFTs 100 are typically arranged on transparent insulating substrate 101 in the form of a matrix array as set forth in the '899 patent. Each TFT 100 includes gate electrode 102 connected to gate address line 113 (see FIG. 2) extending in the row direction, drain electrode 106 connected to drain line 114 extending in the column direction, and source electrode 107 connected to transparent pixel electrode 110 independently formed in the pixel area defined between the array of gate lines 113 and drain lines 114. Pixel electrode 110 operates in conjunction with an opposing common electrode (not shown) on the other side of the liquid crystal layer (not shown) so as to selectively drive the pixel enabling the respective polarizers to transmit or absorb light rays in order to create an image for the viewer. A TFT electrode, to which a data signal is supplied, will be referred to hereinafter as a drain electrode, while the TFT electrode attached to the pixel electrode will be referred to as a source electrode.
More specifically, as shown in prior art FIGS. 1-2, gate electrode 102 of prior art TFT 100 is formed on clear substrate 101. Gate insulating film 103, made of silicon oxide, for example, is formed or deposited on substrate 101 over top of gate electrode 102. Semiconductor film 104, made of amorphous silicon (a-Si), for example, is deposited on substrate 101 over top of gate insulating film 103 and gate 102. Drain and source electrodes 106 and 107 respectively are deposited on substrate 101 over top of layers 103 and 104. The linear-shaped source and drain electrodes are separated from one another by a predetermined distance forming TFT channel 105. Drain and source electrodes 106 and 107 respectively utilize doped a-Si contact layers 106a and 107a in combination with drain-source metal layers 106b and 107b so as to form electrical connections with semiconductor layer 104.
Insulating film 108 is deposited on substrate 101 over the source and drain electrodes to a thickness falling within the range of 2,000 to 8,000 .ANG., preferably about 3,000 .ANG.. Insulating layer 108 may be an organic insulating film obtained by spin-coating and baking polyimide or an acrylic resin, or a silicon oxide inorganic insulating film (SOG film) obtained by spin-coating and baking a silanol compound. Subsequent to the deposition of insulating film 108 on the source and drain electrodes, vias 112 are formed in layer 108 for the purpose of allowing substantially transparent pixel electrodes 110 to contact source electrodes 107. Thus, when a pixel electrode 110 is deposited and patterned over top of insulating layer 108, a portion of it is formed in via 112 as shown in FIG. 1 so as to contact source 107 at point 109. Pixel electrode 110 may be indium-tin-oxide (ITO), for example, and is sputtered on the surface of insulating layer 108 and in via 112 to a thickness of about 1,000 .ANG..
As can be seen in FIG. 2, pixel electrode 110 in certain embodiments of the '899 patent is located completely within the confines of gate line 113 and drain line 114. In other words, as illustrated in FIG. 2, pixel electrode 110 does not overlap either of the address lines. However, it is stated in column 7 of the '899 patent that "transparent electrode 110 may be arranged so as to overlap the drain and gate lines 114 and 113." It is further stated that "hence a maximum effective display area can be obtained . . . an opening ratio of 70% can be realized." In other words, the '899 patent discloses as an alternative embodiment forming pixel electrode 110 so that it overlaps address lines 113 and 114 for the purpose of increasing the pixel opening size or pixel aperture ratio of the AMLCD.
Unfortunately, if pixel electrode 110 of the '899 patent AMLCD is arranged so as to overlap address lines 113 and 114, an undesirably high parasitic capacitance results in the overlap areas between pixel electrode 110 and the address lines. In other words, in the overlap areas, pixel electrode 110 forms a capacitator in combination with the overlapped address lines. The resulting parasitic capacitance C.sub.PL between the pixel electrode 110 and the address lines in the overlap areas is defined as follows: ##EQU1##
where ".epsilon." is the dielectric constant of insulating layer 108, ".epsilon..sub.0 " is a constant value of 8.85.times.10.sup.-14 F/cm, "A" is the area of the resulting capacitor in the overlap area, and "d" is the thickness of insulating layer 108 in the overlap area.
Because of the above-referenced thin profile of insulating layer 108 in the '899 patent, the resulting parasitic capacitance C.sub.PL created by the overlap is undesirably high thereby resulting in capacitive cross-talk in the LCD. Such cross-talk results when the signal voltage intended to be on a particular pixel is not there. Thus, when C.sub.PL is too high, the voltage on the pixel is either higher or lower than intended depending upon how much voltage the other pixels on the signal address line received. In other words, the pixel is no longer satisfactorily isolated when C.sub.PL is too high.
In view of the above with respect to the '899 patent, it is clear that there exists a need in the art for a liquid crystal display having both an increased pixel aperture ratio and reduced capacitive cross-talk in overlap areas so as to simultaneously properly isolate each pixel and increase the pixel openings in order to allow the LCD to be easily used in portable applications with reduced power consumption.
Further with respect to U.S. Pat. No. 5,055,899, the disclosure of this patent does not appreciate the importance of the dielectric constant .epsilon. of insulating layer 108. While referencing numerous materials including SiO.sub.2 which may be used for layer 108, the '899 patent does not discuss either the dielectric constant values .epsilon. of these materials or their importance in helping reduce C.sub.PL in overlap areas. For example, the dielectric constant .epsilon. of SiO.sub.2 (which the '899 patent states may be used as layer 108) is undesirably high (about 3.9) thereby causing C.sub.PL to be too high. When .epsilon. of the insulating layer is too high, capacitive cross-talk results in the display. In view of this, it is apparent that there exists a need in the art for an insulating layer to be disposed between the TFT electrodes (and address lines) and the pixel electrodes, this insulating layer having a dielectric constant value sufficient to reduce C.sub.PL in overlap areas thereby substantially eliminating cross-talk problems in LCDs where the pixel electrodes overlap the address lines.
U.S. Pat. No. 5,182,620 discloses an AMLCD in which the pixel electrodes at least partially overlay the signal lines and an additional capacitor common line thereby achieving a larger numerical aperture for the display.
Unfortunately, the '620 patent suffers from at least the following problems. Firstly, the problems discussed above with respect to the '899 patent are all applicable to the '620 patent. Secondly, due to the additional capacitor common lines of the '620 patent, the pixel aperture ratio of the display is limited to about "48%". This is caused by the AMLCD design where the storage capacitor address lines are separate lines parallel to the column address lines thus resulting in reduced pixel aperture ratios. The AMLCD of the '620 patent uses the additional storage capacitor lines to suppress the effect of C.sub.PL in the overlap area between the pixel electrode and the source. However, the presence of the additional storage capacitor lines is undesirable in that it reduces pixel aperture ratios and increases manufacturing costs.
Additional problems of the AMLCD of the '620 patent are that the pixel aperture ratio is not as great as would otherwise be desired in view of the fact that the pixel electrode does not overlap all address lines, and the required presence of three insulating layers.
It is apparent from the above that there exists a need in the art for an LCD having an increased pixel aperture ratio and substantially no capacitive cross-talk problems which is commercially feasible to manufacture, and a method of making same. Additionally, there exists a need in the art for a TFT and method of making same for implementing the increased pixel aperture ratios and the virtual elimination of cross-talk problems into the display.
It is a purpose of this invention to fulfill the above-described needs in the art, as well as other needs which will become apparent to the skilled artisan from the following detailed description of this invention.